The present invention relates to a wood adhesive useful in manufacturing composite wood products, and is particularly useful in manufacturing hardwood plywood.
Plywood is manufactured by first applying a thermosetting resin adhesive to the veneers comprising the interior plies with a double roll coater (“spreader”), and forming a panel having the desired panel thickness. Next, a plurality of the panels are stacked into a bundle interspersed alternatively with caul plates, and cold pressed in a pre-press. Finally, the cold pressed panels are pressed at an elevated temperature and pressure in a hot press to cure the thermosetting resin adhesive and bond the inner plies and surface veneer layers in each panel together.
In the manufacture of softwood plywood, which is used primarily for structural and exterior uses, the adhesives currently in use typically employ phenol-formaldehyde resins (“PF” resins). PF resins display excellent processing characteristics and low formaldehyde emissions in these applications, but are not of practical significance in interior applications.
Hardwood plywood is primarily used for interior purposes, such as cabinetry, flooring, and wall paneling. These uses are usually decorative and the appearance of the panel is important. The PF resin adhesives used in manufacturing softwood plywood cannot be used effectively in the manufacture of hardwood plywood. This is because hardwood plywood is manufactured with much thinner surface (decorative) veneers (typically 0.016 inch) and an adhesive made with the dark colored PF resin may bleed through these thin veneers detracting from the decorative quality of the surface.
The resin of choice for adhesives used in manufacturing hardwood plywood has historically been urea formaldehyde resins (“UF” resins) because it is colorless, does not show as much bleed through, and is low in cost. Environmental concerns over the emission of free formaldehyde from hardwood plywood manufactured using UF resins has caused many manufacturers to switch to other adhesives, such as those made from polyamide resins or polyvinyl acetate resins. However, adhesives using the latter resins are much more expensive than adhesives made from UF resins.
Some success has been achieved in developing UF resins that have lower amounts of free formaldehyde by lowering formaldehyde-to-urea molar ratios, adding formaldehyde scavengers, or co-polymerizing urea-formaldehyde with other formaldehyde-reactive materials such as melamine or ammonia. However, these modifications result in formaldehyde emissions that are still higher than is optimally desirable, i.e., substantially zero. They also suffer from other disadvantages overcome by the current invention. These include viscosity instability in the mixed adhesive, viscosity instability in the presence of mechanical shear (as in pumps and on the spreader), poor prepress strength and development rate, poor adhesive bond formation, premature dry-out on spread veneers, high spread requirements, poor tolerance to normal moisture variations in the plywood manufacturing process, slower cure in the hot press, and higher costs. Some of these deficiencies were less problematic with the older, high free formaldehyde resins. However, the older technology is no longer environmentally acceptable and would result in plywood that could not comply with current statutes regulating formaldehyde emission.
The wood adhesive of the present invention allows the use of the most advanced low emitting formaldehyde UF resin technology available while overcoming all of the inherent problems associated therewith discussed above. In addition, the wood adhesive of the present invention provides a significant reduction in formaldehyde emissions from said low emitting formaldehyde UF technology by providing formaldehyde emission values of substantially zero when measured by the standard tests accepted by regulatory agencies.